The persistent cart of the buyer or guest: lines, settings (fulfillment, addresses, payment method, shipping rate, coupon, notes), totals, and whether checkout may proceed. With quote: true the store calculates an authoritative quote (shipping rates, tax, credits, fees) and answers quoteFingerprint, which checkout.start submits; a quote that cannot be completed answers the cart with the reason in shippingNotice and issues. A signed-in buyer who still holds a guest token sends guestCart: true to read the guest cart instead. While a checkout attempt is deciding, pendingAttemptId is set and the cart is read-only.
Who may call it: A guest (X-Storefront-Guest) or a signed-in buyer (X-Storefront-Buyer); without either the answer is 401 UNAUTHORIZED.
